Architect Fees for Home Extensions: What You Pay and What You Get

**Percentage-based fees**:

Historically, architect fees were calculated as a percentage of the construction cost — the RIBA standard fee scales were percentage-based until they were withdrawn in the 1990s following OFT competition concerns. Percentage fees are still commonly used, particularly for larger or more complex projects:

• Typical percentage ranges for domestic extension work:
• •Small residential extensions (single-storey, up to £80,000 construction cost): 10–15% of construction cost
• •Medium residential work (multi-storey extensions, refurbishments, £80,000–£250,000 construction cost): 8–12%
• •Larger residential projects: 6–10%

Example: £100,000 extension at 12%: architect fee = £12,000.

*Advantages*: Fee scales automatically with project complexity. If the scope increases during design, the fee adjusts proportionately. *Disadvantages*: Client concern that the architect has no incentive to manage costs down (a higher construction cost = a higher fee). In practice, experienced architects understand that value engineering and cost management are part of their service.

**Fixed fees**:

Fixed fees (a stated total fee for a defined scope of services) are increasingly common for domestic extension work, particularly for smaller practices and design-and-build type services. The fee is agreed in advance for a defined scope:

• Typical fixed-fee packages for a single-storey extension (London, 2025):
• •Planning drawings only (concept design + planning application): £2,500–£6,000
• •Planning + Building Regulations (technical drawings for BR approval): £5,000–£10,000
• •Planning + Building Regulations + Specification (full tender package): £8,000–£18,000
• •Full service (planning + BR + specification + construction monitoring): £12,000–£30,000+

*Advantages*: Client knows the total fee before committing. No surprise increases if the project takes longer to design than expected. *Disadvantages*: If the scope changes significantly (the extension grows from a single-storey to a double-storey), the fixed fee must be renegotiated. Some fixed-fee packages are priced at the minimum for a simple project and the practice charges extras for any complexity.

**Hourly rates**:

• Some architects (particularly for early-stage feasibility work or for limited scope instructions) charge by the hour. London architect hourly rates:
• •Partner/director level: £150–£300/hr
• •Associate/project architect: £90–£150/hr
• •Graduate architect: £60–£90/hr

Hourly rates are appropriate for feasibility studies, boundary or planning advice, and specialist investigations — not for the main design and drawing service, where a fixed fee or percentage is standard.

The RIBA (Royal Institute of British Architects) Plan of Work provides a framework of 8 stages (0–7) that structures the design and construction process. Understanding which stages are included in an architect's appointment is essential for comparing quotes:

**Stage 0 — Strategic Definition**: Initial feasibility and brief development — understanding what the client wants to achieve and whether it is achievable within planning policy, the building's structure, and the available budget. Often included in the initial meeting or as a pre-appointment service.

**Stage 1 — Preparation and Briefing**: Site survey, existing drawings (measured survey of the existing house), preparation of the detailed brief. Most architects charge for the measured survey as a disbursement or as part of the Stage 1 fee.

*Measured survey cost*: £500–£2,000 (or included in the architectural fee for smaller projects; typically outsourced to a specialist measured survey firm for complex properties).

**Stage 2 — Concept Design (Planning Stage)**: Development of the concept design, including: site plan, floor plans showing the new and existing arrangement, elevations, section drawings, planning design statement. The planning drawings are typically prepared at Stage 2 for submission to the LPA.

• *Included in most architectural appointments for home extensions at Stage 2*:
• •Concept design options (2–3 layout options, reviewed with the client before the preferred option is developed)
• •Planning drawings (typically block plan, site location plan, existing and proposed floor plans, existing and proposed elevations, existing and proposed sections)
• •Design and Access Statement (for applications that require it)
• •Planning application submission and management (liaison with the planning officer during determination, responding to requests for additional information)

**Stage 3 — Spatial Coordination (Developed Design)**: Development of the design to a level of detail that allows coordination of structural, mechanical, and electrical elements. At Stage 3, the structural engineer, M&E consultant, and specialist designers are typically engaged and their designs coordinated with the architecture.

• **Stage 4 — Technical Design (Building Regulations)**:
• Preparation of the technical drawings and documents required for Building Regulations approval and for contractor pricing (tender documents). Stage 4 includes:
• •Building Regulations drawings (detailed construction drawings showing structural elements, insulation build-ups, fire safety measures, drainage, etc.)
• •Written specification (describing the materials, standards, and workmanship required)
• •Structural engineer's drawings (coordinated with the architectural drawings)
• •Building Regulations application submission

*Stage 4 is frequently the most under-resourced stage in domestic architectural appointments* — many lower-cost practices produce minimal technical drawings and rely on the contractor to work out the details. The quality of Stage 4 output directly affects the quality of the contractor tender, the competitiveness of the prices received, and the ease of Building Control approval.

**Stage 5 — Manufacturing and Construction (Construction Stage)**: Construction monitoring services — regular site visits by the architect to inspect the progress and quality of the works, issue site instructions, respond to contractor queries, and administer the contract (issue interim certificates, value variations, manage the defects process). Stage 5 is optional in many domestic appointments but provides important quality oversight.

*Construction stage visit cost (if charged separately)*: £100–£300 per visit; or included in the full-service fee. A 20-week project with fortnightly visits = approximately 10 visits.

**Stages 6 and 7 — Handover and Use**: Post-completion activities — issuing the Certificate of Making Good at the end of the defects liability period, coordinating the handover documentation. Rarely included explicitly in domestic appointments.

**Checking an architect's credentials**:

*ARB registration*: All architects practising in the UK must be registered with the Architects Registration Board (ARB) under the Architects Act 1997. The title 'architect' is legally protected — only ARB-registered individuals may call themselves architects. Check the ARB register at arb.org.uk before appointing any practice.

*RIBA membership*: RIBA chartered practices ('RIBA Practice') have passed an assessment of their quality management, professional development, and insurance. RIBA membership is not compulsory (an ARB-registered architect who is not RIBA member is still fully licensed to practice) but is a positive indicator of professional commitment. Check at architecture.com.

*Professional Indemnity Insurance (PII)*: All architects must carry Professional Indemnity Insurance. PII covers the client for any financial loss arising from the architect's negligence — an incorrect drawing, a planning application that fails due to the architect's error, a structural detail that causes a defect. Confirm the PII cover level and that the policy is current before signing the appointment.

**The RIBA Standard Professional Services Contract**:

• Always use a written appointment agreement — and ideally the RIBA Standard Professional Services Contract (RIBA PSC) rather than a one-page letter or email confirmation. The RIBA PSC covers:
• •The scope of services (which RIBA work stages are included)
• •The fee structure (percentage, fixed fee, or hourly)
• •Payment terms (typically monthly in arrears or at defined stage completion dates)
• •The copyright in the drawings (the architect retains copyright but grants a licence for the use of the drawings)
• •Termination provisions
• •Dispute resolution

**Getting comparable quotes from multiple architects**:

• To compare architectural fees fairly:
• •Brief each practice with the same project description — a one-page brief covering the site address, the existing building, the proposed scope, the budget, and the planning position
• •Ask each practice to quote for the same stages (e.g., Stage 2 planning + Stage 4 Building Regulations + Stage 5 construction monitoring — 8 site visits)
• •Ask for a breakdown of the fee by stage — this allows comparison at each stage, not just the total
• •Ask what is not included — measured survey, planning application fees, structural engineer fee, building regulations fee, party wall surveyor

*Planning application fees (LPA charges)*: For a householder planning application in England (2025): £258. This is paid to the LPA, not the architect — but some architects include it in their total quoted fee; others list it as a disbursement.

*Building Regulations application fees*: For a full plans application (house extension), the LPA Building Control fee is typically £200–£400 for a single-storey extension; £400–£800 for a loft conversion or larger scheme. Private Approved Inspector fees vary. This is a separate cost from the architect's fee.